Day-lighting is the mechanism of providing controlled admission of natural light into a space through windows to reduce or eliminate electric lighting usage. The benefits from architectural day-lighting are maximized when photo-sensors are used to control the electric lighting system. Photo-sensors estimate the amount of light in their field of view and enable automatic adjustment of the output level of electric lights based on the amount of light detected through control devices. The estimation of the illumination level in a room allows utilization of day-lighting when available and increases the lighting levels only when required, while keeping the illumination level of the space constant. Threshold on and off values can be set to respond to specific lighting conditions. Based on the estimated illumination level, the photo-sensor can operate on/off switching of various lights or a continuous dimming system for greater user satisfaction.
While existing photo-sensors have large fields of view, their operation is limited by the fact that their output is based on the total amount of light in their field of view. This mode of operation is especially disadvantageous when the task-area does not directly receive the light and is darker as compared to its surrounding areas. Similarly, if there are multiple task-areas in a given space, multiple photo-sensors are required to independently control the lighting of these areas, thus increasing the cost and complexity of the control systems. It is desirable that a single sensor measure the illumination levels (both global and local) of different areas in a space independent from each other and to control multiple devices accordingly.